1. Field of the Invention
The present invention relates to electrically heated subsea pipelines. More particularly, method and apparatus are provided for determining the electrical integrity of subsea pipelines that are equipped with wet-mateable subsea connectors for electrical heating of the pipeline.
2. Description of Related Art
Companies seeking to recover hydrocarbons offshore must often drill wells in water several thousands of feet deep. In many cases, groups of these deep wells feed recovered hydrocarbon fluids into remote platforms via lengthy underwater pipelines resting on the ocean floor. Because these subsea pipelines lay at great depths, the seawater that surrounds them has a temperature in the range of 40° F. The hydrocarbon fluids, however, usually reach the ocean floor from the wells at much greater temperatures. That is, the hydrocarbons arrive at the high temperatures near those typical of depths of thousands of feet below the ocean floor. Thus, the hydrocarbon fluids cool dramatically once they reach the ocean floor. The hydrocarbons are produced with at least small amounts of water. As this water and hydrocarbon fluid mix cools, it can undergo changes that decrease its flow rate through the subsea pipelines. The viscosity of some crude oils increases severely when the oil cools. Other crude oils deposit paraffin on the pipeline walls as they cool. Light hydrocarbon gases under pressure can form crystals called “hydrates” when mixed with water. If these hydrates or paraffin deposits plug pipelines, they can be quite difficult to remove. Hydrate removal methods that work in shallow waters are often ineffective in deep waters. High pressure in the pipeline and uneven ocean floor topography only compound the problem. Any of the problems caused by low seawater temperatures can result in expensive losses in production.
Usually, hydrocarbon fluids move rapidly enough through the pipeline to prevent plug formation. However, if well production stops or slows, plugs may develop. Pipeline operators sometimes heat subsea pipelines to warm recovered hydrocarbon fluids if they reach problematically low temperatures. Bundling pipelines with a separate line of circulating heating fluid has long been practiced in the industry. Heating via electrical methods has also become possible. One such electrical heating method utilizes a pipe-in-pipe pipeline design. An inner pipe carries the recovered hydrocarbons. An outer “casing” pipe concentrically surrounds this inner pipe. The two pipes are electrically connected at one or both ends. Voltage is applied at the opposite end or at the midpoint. Alternating current runs on the exterior surface of the inner pipe and along the interior surface of the casing pipe. The annulus between the pipes contains electrically insulative centralizers and panels. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707, issued Nov. 7, 2000, which is incorporated by reference herein. Another configuration for electrical heating is the Single Heated Insulated Pipe (“SHIP”) method. In this configuration, power flows along the electrically insulated pipeline and returns through sea water around the line. This method is disclosed in U.S. Pat. No. 6,049,657, which is also incorporated by reference.
Power is only supplied to electrical heating systems for pipelines as necessary to ease hydrocarbon flow. Heating may not be needed until years after initial pipeline construction. Furthermore, once a plug loosens or melts, heat is no longer necessary. Only a segment of a pipeline may require attention. Therefore, apparatus and methods have been developed for deploying pipelines in a configuration called “electrical-ready.” Apparatus is provided when the pipeline is deployed to allow a source of electrical power to be applied to a selected segment of the line when heating is needed at that location. This allows considerably lower investment costs and adds flexibility to the operation of pipelines. The apparatus and methods for making subsea pipelines ready for electrical heating (i.e., “electrical-ready”) are disclosed in U.S. Pat. No. 6,371,693, which is commonly assigned and incorporated by reference herein. This patent teaches that various configurations of electrically heated pipelines can be made electrical-ready.
Before high voltage (thousands of volts) from a high current source (in excess of 1000 amperes) is applied to heat an electrical-ready subsea pipeline, however, it is preferable that the electrical integrity of the pipeline be known. Any number of events may cause a short circuit in the pipeline. For example, water may leak through the casing (outside) pipe into the space between the casing and the carrier pipe. Short circuits may even result from gross damage to the pipeline that does not actually cause a water leak, such as dents or bends in the casing pipe that cause it to touch the inner pipe or become close enough to allow breakdown when high voltage is applied. Electrical integrity may be compromised from the very beginning of the pipeline's lifetime, during construction because of entrapment of moisture in the annulus. For example, water may enter the segments of the pipeline as they travel to the ocean floor, or as they are welded together. Electrical short circuits may also arise from metallic contamination incurred during the welding process that shorts the two pipes. A contamination event may be sufficient to cause an electrical breakdown when high voltage is applied even if contaminants do not physically bridge the gap between the inner and casing pipelines. Therefore, a reliable electrical integrity check is preferred before any voltage is applied. This may be as the pipeline is installed, to be sure that it is operable at that time, or after a period of operation of the pipeline when heating becomes needed.
A method called “Time Domain Reflectometry,” or “TDR,” is generally used to locate electrical faults within transmission lines such as cables. Equipment operators apply electrical pulses to the transmission line. The operators then measure how long it takes before reflections of the pulses caused by different structures in the line to return to the input source. Defects in the transmission line cause reflections. The reflection return time directly relates to the location of the structure or defect that caused the reflection. TDR techniques are known in industry and are used to locate defects in cables with a reasonable degree of accuracy (+/−20 feet).
Known electrical integrity checks for pipelines often rely on electrical cables running through the length of the pipeline. For example, U.S. Pat. No. 5,305,798, issued Apr. 26, 1994, uses an encapsulated conductor cable attached to the inner surface of a casing pipe to detect leakage defects. This method only indirectly measures the physical integrity of the pipeline because it actually measures the physical integrity of the cable. Supplying such a cable may be prohibitively expensive for lengthy pipelines and may prevent the line from routine cleaning using gauge or foam pigs. Furthermore, if the pipeline is damaged, the cable may also require repair. Transmission line repair only further increases costs. Also, this system detects leaks, but not various other problems that affect subsea pipelines. At very deep water depths, a repair is deemed almost technically impossible without complete recovery of the flowline to the surface.
U.S. Pat. No. 5,905,194, issued May 18, 1999, discloses a fault detection system that does not require a separate transmission line. This system uses the pipes themselves as electrical conductors. This system, however, requires multiple electrical connections throughout the length of the pipeline. Furthermore, these connections—and the fault detection system in general—are not designed for underwater use. Deepwater pipelines require components specifically constructed to withstand the harsh conditions of the ocean floor environment, especially the large hydrostatic pressure. Furthermore, because this system is not for underwater applications, it detects leaks from inside the pipeline, rather than leaks from outside the pipeline. Therefore, there is a need for an electrical integrity check system fully adapted for use with electrical-ready subsea pipelines. This system should not require a separate signal transmission line or multiple connections along the pipeline.
A commonly assigned application for “Apparatus and Method for Electrical Testing of Electrically Heated Pipe-in-Pipe Pipeline,” filed Jul. 20, 2001, Ser. No. 09/910,295, discloses method and apparatus for testing a pipeline before, during and after subsea installation. In one embodiment, a time domain reflectometer (TDR) is electrically connected to the inner pipe of a pipe-in-pipe pipeline and a voltage pulse of less than about 100 volts is applied to the pipeline.
There is a need for an apparatus for making electrical integrity tests of electrically heated subsea pipelines that are “electrical-ready,” that is, that are installed and equipped for applying electrical power to a selected segment of the pipeline when it is needed. The electrical integrity apparatus should be operable from a surface location and should provide an indication of the location of a defect in the pipeline. Methods for employing the apparatus are needed to increase the reliability of such pipelines.